1891.] liquid electrodes in vacuum tubes. 229 



the original gas or be derived from the cathode by the dis- 

 integration which has been shown to accompany the passage of 

 the discharge, and in many cases at least to follow the incidence of 

 ultra-violet light. 



Whatever the nature of the discharge may be it certainly does 

 not follow Ohm's law. Hittorf and Homen, using a current which 

 outside the vacuum tube seemed steady and continuous, found 

 under certain limitations — depending on the spread of the ne- 

 gative glow over the cathode — that the potential difference at 

 low pressures between the electrodes was nearly independent of 

 the strength of the current. At very low pressures the fall of 

 potential took place in great measure quite close to the cathode 

 surface. Hittorf 1 found in the positive part of the discharge a 

 more or less regular fall of potential, and this fall per unit length 

 of tube was much greater than that in the non-luminous Faraday 

 space. Thus here at least non-luminosity seems to indicate 

 diminished resistance. 



As one of the liquids I tried was mercury some points con- 

 nected with discharge through Hg. vapour claim special notice. 

 Mercury vapour being usually considered mon-atomic, it is clear 

 that discharge through it presents some peculiarities on the 

 electrolytic theory of discharge, there being no very obvious 

 means of separating the gas into carriers of positive and negative 

 electricity. 



This peculiarity drew to it the attention of Professor 

 Schuster 2 who states as the result of very careful experiments: 

 — "I find that if the mercury vapour is sufficiently free from 

 air, the discharge through it shows no negative glow, no dark 

 spaces, and no stratifications." He also found the discharge to 

 present almost exactly the same features at both electrodes. The 

 introduction of a very slight trace of air set up the Crookes' 

 dark space at once. Experiments in agreement with Schuster's 

 are described by Natterer 3 . Professor Crookes * has, however, 

 arrived at results diametrically opposed to those of Professor 

 Schuster. After taking the greatest pains to prevent the pre- 

 sence of any foreign gas, he found a distinct Crookes' space and 

 at least traces of a dark Faraday space. He employed aluminium 

 electrodes, while Professor Schuster preferred them of platinum 

 with only a small surface exposed. 



Elster and Geitel 5 examining the effect of the presence of 

 a heated wire in a tube on the contained gas, found mercury 



1 Wied. Ann. 20, 1883, pp. 726 et seq. 



2 Proceedings of the Royal Society, Vol. xxxvu., 1884, p. 319. 



3 Wied. Aim. 38, 1889, p. 669. 



4 Journal of Electrical Engineers, Vol. xx., Feb. 1891, pp. 44 — 6. 



5 Wied. Ann. 37, 1889, pp. 319 and 327. 



18—2 



